Density (cooling)

Density as a function of depth in the modern wintertime Density as a function of depth in the modern wintertime Antarctic and changes in this density structure for Antarctic and changes in this density structure for uniform changes in seawater temperatureuniform changes in seawater temperature. Cooling . Cooling entire water column nearly doubles the vertical density entire water column nearly doubles the vertical density difference.difference.

Effect of Stratification on PCO2

Global cooling an important factor promoting Global cooling an important factor promoting high-latitude stratificationhigh-latitude stratification

Polar ocean stratification prevents deep ocean Polar ocean stratification prevents deep ocean ventilationventilation Traps more carbon in the deep seaTraps more carbon in the deep sea

During Quaternary climatic cyclesDuring Quaternary climatic cycles Interglacial periods sufficiently warm to allow Interglacial periods sufficiently warm to allow

deep water convection in Antarcticdeep water convection in Antarctic Not the North PacificNot the North Pacific

During glacial periodsDuring glacial periods Stratification in Antarctica and North PacificStratification in Antarctica and North Pacific Contribute to lower atmospheric COContribute to lower atmospheric CO22

Phasing of Insolation and Ice Volume

41,000 and 23,000 year 41,000 and 23,000 year components of ice volumecomponents of ice volume Lag behind N. Lag behind N.

Hemisphere insolation Hemisphere insolation by physically by physically reasonable amountreasonable amount

41,000 year S. 41,000 year S. Hemisphere cycles has Hemisphere cycles has same lagsame lag 23,000 year ice volume 23,000 year ice volume

leads S. Hemisphere leads S. Hemisphere summer insolation summer insolation forcingforcing

Unreasonable Unreasonable relationshiprelationship

Questions? If northern hemisphere glaciation drives If northern hemisphere glaciation drives

the benthic the benthic 1818O record andO record and Northern hemisphere summer insolation Northern hemisphere summer insolation

leads the ice volume signal andleads the ice volume signal and The precessional cycle is out of phase in The precessional cycle is out of phase in

the hemispheresthe hemispheres How can polar ocean stratification affect How can polar ocean stratification affect

atmospheric COatmospheric CO22?? When precessional insolation forces ice When precessional insolation forces ice

growth in the northern hemisphere, will it growth in the northern hemisphere, will it not keep the southern hemisphere warm? not keep the southern hemisphere warm?

No 23 kyr forcing of CO2

Ice core COIce core CO22 record shows minimal power record shows minimal power at 23 or 41 kyr?at 23 or 41 kyr?

Perhaps one does not need to understand Perhaps one does not need to understand specifically the role of the 23 kyr cycle in specifically the role of the 23 kyr cycle in COCO22 change? change?

Orbital-Scale Changes in CO2

COCO22 record from Vostok record from Vostok Interglacial maxima 280-Interglacial maxima 280-

300 ppm300 ppm Glacial minima 180-190 Glacial minima 180-190

ppmppm 100,000 year cycle dominant100,000 year cycle dominant Match ice volume recordMatch ice volume record

Timing Timing AsymmetryAsymmetry

Abrupt increases in COAbrupt increases in CO22 match rapid ice meltingmatch rapid ice melting

Slow decreases in COSlow decreases in CO22 match slow build-up of icematch slow build-up of ice

Orbital-Scale Changes in CO2 Vostok 150,000 recordVostok 150,000 record

23,000 and 41,000 23,000 and 41,000 cyclescycles

Match similar cycles Match similar cycles in ice volumein ice volume

Agreement suggests Agreement suggests cause and effect cause and effect relationshiprelationship Relationship Relationship

unknownunknowne.g., does COe.g., does CO22 lead lead

ice volume?ice volume? Correlations not Correlations not

sufficient to provide sufficient to provide definite evaluationdefinite evaluation

Spectral Properties of the Vostok Time Spectral Properties of the Vostok Time SeriesSeries

Frequency distribution of Frequency distribution of power spectrumpower spectrum

Vertical lines correspond Vertical lines correspond to periodicities of 100, 41, to periodicities of 100, 41, 23 and 19 kyr23 and 19 kyr Danny is mostly Danny is mostly

correct!correct! Position of 23 and 19 kyr Position of 23 and 19 kyr

spectral peaks affected by spectral peaks affected by uncertainties in timescaleuncertainties in timescale Sensitivity test of Sensitivity test of

spectral analysis spectral analysis Position and strength Position and strength

of 100- and 40-kyr-of 100- and 40-kyr-spectral peaks not spectral peaks not affectedaffected

Spectrum significantly Spectrum significantly modified for modified for periodicities <30 kyrperiodicities <30 kyr (from Petit (from Petit et alet al., 1999, ., 1999, NatureNature 399:429- 399:429-

436)436)

Alternative Explanation & Reality

Lower 23 kyr solar insolation in the N. Lower 23 kyr solar insolation in the N. hemisphere and increased albedo from hemisphere and increased albedo from expansion of N. hemisphere glaciersexpansion of N. hemisphere glaciers Drops whole-earth temperature enough Drops whole-earth temperature enough

to promote year-round Southern Ocean to promote year-round Southern Ocean stratificationstratification

No one understands the 23 kyr cycle No one understands the 23 kyr cycle connection with Antarctic warmingconnection with Antarctic warming

We still lack a complete mechanistic We still lack a complete mechanistic sequence, going from forcing, to sequence, going from forcing, to amplifiers, to glacial cyclesamplifiers, to glacial cycles

Why is the 100K year Cycle a Mystery?

Appeared about 0.9 my and became the Appeared about 0.9 my and became the dominant climate cycledominant climate cycle

Insolation forcing at 100,000 years is negligibleInsolation forcing at 100,000 years is negligible Earth’s orbital parameters did not changeEarth’s orbital parameters did not change

41,000 and 23,000 year cycles continue41,000 and 23,000 year cycles continueBasic character of insolation cycles not Basic character of insolation cycles not

changed over >2.75 mychanged over >2.75 my Key questionsKey questions

Why did more ice accumulate after 0.9 mya?Why did more ice accumulate after 0.9 mya? Why did these large ice sheets melt rapidly Why did these large ice sheets melt rapidly

every 100,000 years?every 100,000 years?

Larger Ice Sheets Long cores show gradual coolingLong cores show gradual cooling

Marine Marine 1818O recordO record N. Hemisphere ice accumulationN. Hemisphere ice accumulation

Important only after 2.75 myImportant only after 2.75 my Cooling trend results from Cooling trend results from

tectonic-scale changetectonic-scale changeCOCO22 – weathering/volcanism – weathering/volcanism

Orbital scale cycles Orbital scale cycles superimposed on long term superimposed on long term changechangeFits the small-glacial to large Fits the small-glacial to large

glacial phase modelglacial phase model

Ice Sheets Over Last 150,000 y 100,000 year cycle 100,000 year cycle

dominantdominant 23,000 and 41,000 23,000 and 41,000

year cycles presentyear cycles present Two abrupt glacial Two abrupt glacial

terminationsterminations130,000 yeas ago130,000 yeas ago~15,000 years ~15,000 years agoago

Is the 100,000 year Is the 100,000 year cycle real?cycle real?

Insolation at 65°N Varies entirely at periods ofVaries entirely at periods of

Axial tilt (41,000 years)Axial tilt (41,000 years) Precession (mainly 23,000, also 19,000 years)Precession (mainly 23,000, also 19,000 years)

Insolation at 65°N

But there is noBut there is no100,000 year100,000 yearinsolation cycle!!!insolation cycle!!!

??

Alternate Explanation Character of ice movement may have changedCharacter of ice movement may have changed

Glacial moraines dated at ~2 my found quite Glacial moraines dated at ~2 my found quite far south in N. Americafar south in N. AmericaSuggesting that ice sheets thinSuggesting that ice sheets thin1818O data indicate they were small volumeO data indicate they were small volume

Slip-Sliding Glaciers Ground on which ice accumulated may Ground on which ice accumulated may

have been differenthave been different If no ice before 2.75 myIf no ice before 2.75 my

Thick accumulations of soilThick accumulations of soilSoils saturated with water and ice Soils saturated with water and ice began slip-sliding under heavy weightbegan slip-sliding under heavy weight•Deformation at base of glacier fastDeformation at base of glacier fast•Faster than internal deformationFaster than internal deformation

If they slid towards southern latitudesIf they slid towards southern latitudes•Ablation rates would keep volume Ablation rates would keep volume lowlow

Younger Fatter Glaciers Ice sheets erode landscapesIce sheets erode landscapes

Remove soil horizonsRemove soil horizonsLeaving bare bedrockLeaving bare bedrock

No soft-sediment deformationNo soft-sediment deformationGlaciers can grow thick don’t melt as easilyGlaciers can grow thick don’t melt as easily

Why Rapid Deglaciation? Why at 100,000 years?Why at 100,000 years?

Intuitively, must be Intuitively, must be summer insolationsummer insolation

Major control on size of Major control on size of glaciersglaciers

Insolation control at Insolation control at 100,000 years miniscule100,000 years miniscule

Strong summer insolation Strong summer insolation peaks at precession cyclepeaks at precession cycle Resulting from Resulting from

eccentricity modulationeccentricity modulationMatch rapid glacial Match rapid glacial

terminationsterminations

Rapid deglaciationsRapid deglaciationswhen eccentricitywhen eccentricitymodulations createmodulations createlarge summer insolationlarge summer insolation

Hypothesis Rapid ice melting at 100,000 cycleRapid ice melting at 100,000 cycle

Is created from 23,000 year insolation curveIs created from 23,000 year insolation curve It is possible because ice sheet meltingIt is possible because ice sheet melting

Sensitive to only one side of the modulation Sensitive to only one side of the modulation envelopeenvelopeProminent cooling on the other side of the Prominent cooling on the other side of the

envelopeenvelope•Are irrelevant to ice sheet meltingAre irrelevant to ice sheet melting•Enhance growth by a small amountEnhance growth by a small amount

Ice sheets grow large during large glaciation Ice sheets grow large during large glaciation phasephase But melt rapidly every 100,000 yearsBut melt rapidly every 100,000 years

Problem with Observations Eccentricity modulates precessional signal at Eccentricity modulates precessional signal at

413,000 years413,000 years Producing unusually large summer insolationProducing unusually large summer insolation Most recent large peaks at 200,000 and Most recent large peaks at 200,000 and

600,000 years ago600,000 years agoRapid ice sheet melting expectedRapid ice sheet melting expectedYet none are observedYet none are observed

Other insolation-observation mismatchesOther insolation-observation mismatches Particularly at 400,000 years ago and at Particularly at 400,000 years ago and at

10,000 years ago10,000 years agoMismatch could be partly explained by Mismatch could be partly explained by

critical internal interactions in climate critical internal interactions in climate systemsystem

The Bigger they are… Did the 100,000 year cycle follow appearance Did the 100,000 year cycle follow appearance

of large ice sheets?of large ice sheets? Did large ice sheets produce internal Did large ice sheets produce internal

interactions?interactions? Produced positive feedbacks that destroyed Produced positive feedbacks that destroyed

large glaciers every 100,000 yearslarge glaciers every 100,000 years Large bedrock depressionsLarge bedrock depressions

When ice meltedWhen ice meltedRetreated to lower elevationsRetreated to lower elevations

•Kept ice sheets warmKept ice sheets warm Models show that large ice sheets rebound Models show that large ice sheets rebound

faster than small ice sheetsfaster than small ice sheets

Other possibilities?? Marine ice sheets on continental shelvesMarine ice sheets on continental shelves

Melt faster than large continental ice Melt faster than large continental ice sheetssheetsRising sea level?Rising sea level?

Continental margin ice sheet meltingContinental margin ice sheet meltingHelped melt continental ice sheets?Helped melt continental ice sheets?

Rising CORising CO22 levels would hasten glacial ice levels would hasten glacial ice meltingmelting Strong correlations between ice volume Strong correlations between ice volume

and COand CO22 levels levelsRelationship possibleRelationship possible

Conceptual Model SPECMAP results of Imbrie SPECMAP results of Imbrie et alet al..

Insolation changes drove ice sheets at 41K and Insolation changes drove ice sheets at 41K and 23K cycles23K cyclesSignal quickly transferred climate system by Signal quickly transferred climate system by

winds, sea level and deep water circulationwinds, sea level and deep water circulation

SPECMAP Model As ice sheets exceeded a critical size thresholdAs ice sheets exceeded a critical size threshold

Large size of ice became a major factorLarge size of ice became a major factorProducing many more ice-driven responsesProducing many more ice-driven responses

•Strongest may have been deep water Strongest may have been deep water circulation control on COcirculation control on CO22 levels levels

Deep Water Circulation Large N. Hemisphere ice sheetsLarge N. Hemisphere ice sheets

Altered low-level winds in regionsAltered low-level winds in regions N. Atlantic deep water formationN. Atlantic deep water formation

Altering the timing of COAltering the timing of CO22 cycle cycle

•Providing positive feedbacks to ice Providing positive feedbacks to ice sheet growthsheet growth

N. Hemisphere ice sheets control N. N. Hemisphere ice sheets control N. Atlantic deep water formationAtlantic deep water formationN. Atlantic deep water formation N. Atlantic deep water formation exerts controls on COexerts controls on CO22

N. Ice Drives S. Climate Since response of deep water circulation faster Since response of deep water circulation faster

than ice sheet responsethan ice sheet response Ocean responses in the S. Hemisphere lead Ocean responses in the S. Hemisphere lead

changes in N. Hemisphere ice sheet volumechanges in N. Hemisphere ice sheet volumeEven though both are responding to an Even though both are responding to an

initial change in N. Hemisphere climateinitial change in N. Hemisphere climate Critics of SPECMAP hypothesisCritics of SPECMAP hypothesis

Point to mismatch between COPoint to mismatch between CO22 and ice and ice volume or S. Hemisphere temperaturevolume or S. Hemisphere temperatureAt 115,000 and 75,000 years agoAt 115,000 and 75,000 years agoCorrelations with Southern Hemisphere Correlations with Southern Hemisphere

temperature now shown to be excellenttemperature now shown to be excellent

Other Explanations Southern Ocean carbon cycle reacted to Southern Ocean carbon cycle reacted to

orbital forcingorbital forcing Earlier and independently of N. Earlier and independently of N.

Hemisphere ice sheetsHemisphere ice sheetsProblems with timingProblems with timing

•Linked to orbital forcing Linked to orbital forcing independently of ice sheetsindependently of ice sheets–Yet with a similar overall timingYet with a similar overall timing

Resonant response – a characteristic Resonant response – a characteristic response timing regardless of the forcingresponse timing regardless of the forcing Why did it begin suddenly at 0.9 mya?Why did it begin suddenly at 0.9 mya?

Summary Really quite close to understanding ice sheet Really quite close to understanding ice sheet

variationsvariations 2.75 – 0.9 mya2.75 – 0.9 mya

Ice sheets controlled by summer insolationIce sheets controlled by summer insolation•At rhythms of 41K and 23K yearsAt rhythms of 41K and 23K years

– Just as Milankovitch predictedJust as Milankovitch predicted 0.9 mya global cooling allowed growth of 0.9 mya global cooling allowed growth of

large ice sheetslarge ice sheetsDominant 100K rhythm paced by summer Dominant 100K rhythm paced by summer

insolationinsolation•Governed by internal feedbacks Governed by internal feedbacks

produced by ice sheetsproduced by ice sheets